Elastic and inelastic transmission in guided atom lasers: a truncated Wigner approach

We study the transport properties of an ultracold gas of Bose-Einstein condensate that is coupled from a magnetic trap into a one-dimensional waveguide. Our theoretical approach to tackle this problem is based on the truncated Wigner method for which we assume the system to consist of two semi-infinite non-interacting leads and a finite interacting scattering region with two constrictions modelling an atomic quantum dot. The transmission is computed in the steady-state regime and we find a good agreement between truncated Wigner and Matrix-Product State calculations. We also identify clear signatures of inelastic resonant scattering by analyzing the distribution of energy in the transmitted atomic matter wave beam.Comment: 10 pages, 5 figures, submitted to Phys. Rev.

Coherence and many-body effects in the transport of Bose–Einstein condensates

This thesis presents investigations on the interplay of coherence and many-body effects in the quasi one-dimensional transport of Bose-Einstein condensates (BEC) through scattering potentials. Such configurations can be realized with guided atom lasers that provide a coherent atomic beam. An exact theoretical description of the dynamics is out of reach due to the presence atom-atom interactions. Different levels of approximations are nevertheless possible with their strengths and weaknesses. The mean-field approximation, where the dynamics of the BEC is governed by the Gross-Pitaevskii equation, is most commonly used in the field of ultracold atoms. In this thesis the truncated Wigner method is used to go beyond the standard Gross-Pitaevskii description. This method is adapted in order to study the scattering of Bose-Einstein condensates in one-dimensional waveguides where atom-atom interactions and external potentials are nonvanishing only in a finite region of space. In this case, the truncated Wigner method is combined with the smooth exterior complex scaling method and incorporates quantum noise that originate from the vacuum fluctuations in the waveguide. Inelastic scattering is shown to play a major role in the resonant transport of BEC through a symmetric double potential barrier effectively forming an atomic quantum dot. Indeed, fully resonant transmission is prohibited and incoherent atoms as well as collective oscillations are detected in the transmitted beam. It is also shown that inelastic scattering destroys Anderson localization in the case of transport through disordered potentials. The classical (incoherent) ohmic transmission is recovered for finite atom-atom interactions. The validity of the truncated Wigner method is then assessed using the semiclassical van Vleck-Gutzwiller propagator in the Fock space of the many-body system. It is shown that the truncated Wigner method corresponds to the so-called diagonal approximation, and it is possible to identify the leading correction to the truncated Wigner results, which is provided by the so-called coherent backscattering (CBS) contribution. Coherent basckattering in Fock space is a genuine quantum many-body effect that lies beyond the reach of any mean-field approach. For the case of closed Bose-Hubbard models, the relevance of CBS is confirmed by numerically comparing the (classical) truncated Wigner evolution probabilities to the exact quantum probabilities in Bose-Hubbard models: While a CBS-induced enhancement of the return probability to the initial state is clearly seen in the exact quantum simulations of the bosonic many-body system, this enhancement is absent in the classical calculations. The magnitude and dependence of the CBS contribution on gauge fields, which break time-reversal invariance, is numerically confirmed. For the case of disordered open systems, it can be shown that this contribution as well as next-to leading order contributions vanish thereby confirming the validity of the truncated Wigner method.Cette thèse présente les investigations réalisées sur la relation entre les effets à plusieurs corps et la coherence lors de phénomènes de transport quasi-unidimensionels de condensats de Bose-Einstein (CBE) à travers divers potentiels de diffusion. De telles configurations peuvent être réalisées avec des lasers à atomes qui créent des faisceaux coherents de matière. Une description exacte de la dynamique à plusieurs corps est hors de portée à cause des interactions entre atomes. Malgré tout, différents niveaux d'approximation sont possibles, chacun possédant leurs forces et faiblesses. L'approximation en champ moyen, où la dynamique du CBE est régie par l'équation de Gross-Pitaevskii, est la plus utilisée dans le domaine des atomes ultrafroids. Dans cette thèse, la méthode de Wigner tronquée, qui est une amélioration de la description fournie par l'équation de Gross-Pitaevskii, est utilisée. Cette méthode est adaptée pour l'étude de la diffusion de condensats de Bose-Einstein dans des guides d'ondes unidimensionels où les interactions entre atomes et les potentiels extérieurs ont un support fini. Dans ce cas, la méthode de Wigner tronquée est combinée avec la méthode de dilatation complexe extérieure lissée (smooth exterior complex scaling) et avec du bruit quantique, provenant des fluctuations quantiques dans le guide d'onde. Les effets, non négligeables, des collisions inélastiques sont mis en évidence lors du transport résonnant d'un CBE à travers une double barrière de potentiel symétrique formant un point quantique. En effet, une transmission résonnante parfaite est prohibée et des atomes incoherents ainsi que des oscillations collectives sont détectées dans le faisceau atomique transmis. Il est aussi démontré que la diffusion inélastique est responsable de la destruction de la localisation d'Anderson dans le cas du transport à travers des potentiels désordonnés. Une transmission ohmique (incohérente) est retrouvée pour des interactions finies entre atomes. La validité de la méthode de Wigner tronquée est évaluée en utilisant le propagateur semiclassique de van Vleck-Gutzwiller dans l'espace de Fock du sytème à plusieurs corps. Il est démontré que la méthode de Wigner tronquée correspond à l'approximation diagonale et qu'il est possible d'identifier la correction dominante appellée rétrodiffusion cohérente. La rétrodiffusion cohérente dans l'espace de Fock est un effet à plusieurs corps purement quantique et ne peut être reproduite par aucune théorie en champ moyen. L'analyse numérique des probabilités de transitions quantiques pour des systèmes de Bose-Hubbard confirme la présence de rétrodiffusion cohérente alors qu'elle est absente pour des simulations utilisant la méthode (classique) de Wigner tronquée. L'amplitude ainsi que la dépendance vis-à-vis d'un champ de jauge de la contribution de rétrodiffusion cohérente est confirmée numériquement. Dans le cas de systèmes ouverts désordonnés, il a été montré que la rétrodiffusion cohérente ainsi que les corrections d'ordre supérieur sont inexistantes, confirmant la validité de la méthode de Wigner tronquée

DEVELOPMENT OF NEW ITU APPROACH TO INTERNATIONAL FREQUENCY SPECTRUM MANAGEMENT AND STANDARDIZATION PROCESSES

Mednarodna Unija za telekomunikacije (ITU) je specializirana organizacija Združenih Narodov za telekomunikacije s sedežem v Ženevi, ki med drugim skrbi za učinkovito in usklajeno upravljanje z radiofrekvenčnim spektrom (10kHz-3000GHz) in to na način, da so zagotovljene pravice in enake možnosti njegovega izkoriščanja vsem uporabnikom, državam članicam (193). Vse od njene ustanovitve leta 1865 je tehnološki napredek motiviral oblikovanje ITU mednarodnih standardov za izgradnjo in funkcioniranje telekomunikacijskih sistemov in omrežij in kasneje tudi za izkoriščanje radiofrekvenčnega spektra. Tehnološki razvoj je uravnaval dinamiko aktivnosti in pogojeval organizacijske spremembe in statutarne reforme. Strokovna priporočila pa so postala univerzalna referenca za uspešen razvoj in standardizacijo telekomunikacijskih sistemov. Pa vendar je že nekaj časa opaziti, da je učinkovitost ITU v vse večjem razkoraku z evolucijo in preobrazbo telekomunikacijskega okolja, pri čemer ni skrivnost, da so številni poskusi zadnjih let, kako uskladiti strukturo in delovanje ITU z dinamičnim razvojem okolja, bili neuspešni. Inertnost, pa tudi oportunizem različnih subjektov, je bila in je še vedno močna ovira, da bi se ITU uspešno spopadala z izzivi časa. Radiofrekvenčni spekter in orbitalne pozicije, ki pripadajo različnim radiofrekvenčnim alokacijam, so edinstvena naravna dobrina, katere usklajena in transparentna mednarodna regulativa je permanentna skrb vsake suverene države. Učinkoviti procesi standardizacije v globalnem okolju pa so ključ za uspešno pot k hitremu razvoju telekomunikacijske infrastrukture v času, ko je tehnološka konvergenčnost izbrisala meje med tehnologijo, telekomunikacijskimi sistemi in telekomunikacijskimi aplikacijami. Disertacija ponuja podrobno analizo aktivnosti ITU na dveh osrednjih segmentih, t.j. radiokomunikacije in standardizacija. Na osnovi dolgoletnega opazovanja in proučevanja dela in učinkovitosti ITU, zaključki nedvoumno kažejo na potrebo po strukturnih reformah, pot do katerih pa je zaradi politične dimenzije, ki je vedno prisotna v medvladnih organizacijah, izjemno zapletena. V sodobnem telekomunikacijskem okolju, kjer tehnološki razvoj prehiteva počasne vladne odločitve, so potrebne jasne strateške orientacije in trdna odločenost držav članic, da se pristopi k vsebinskim spremembam. S tem namenom je predstavljen tudi predlog novega konstitutivnega in organizacijskega modela ITU, katerega implementacija bi omogočila boljše pogoje za doseganje zastavljenih ciljev. Sedanje težave v funkcioniranju te specializirane organizacije OZN so v največji meri posledica njenega zaostalega organizacijskega koncepta, ki sodi v zgodovinsko okolje. Države članice imajo še vedno izključno vlogo pri pomembnih odločitvah, žal pa njihove vlade ne premorejo več strokovnega potenciala za doseganje znanstvenega napredka. V ta prostor so vstopili drugi kreativni deležniki (iz industrije in nevladnih organizacij) ki so postali gonilna sila vsega razvoja. To so tisti ki želijo ustvarjati v drugačnem okolju, kjer bodo suvereni v odločanju na področjih njihove izključne kompetentnosti. Pogosto zapletena razmerja med državami članicami z različnimi interesi, velikimi in malimi, razvitimi in manj razvitimi, sposobnimi in odsotnimi, egoističnimi in kooperativnimi – onemogočajo univerzalne rešitve, ko gre za učinkovito in transparentno uporabo radiofrekvenčnega spektra. Na področju standardizacije ICT sistemov in aplikacij so razmere še bolj zapletene, saj so ti procesi v zadnjih desetletjih spremenili vsebino zaradi potreb po hitrem regionalnem razvoju in predvsem zaradi hitrega tehnološkega napredka, kateremu tradicionalni in klasično počasni ITU pristop k standardizaciji ni mogel več slediti. Dinamika razvoja pa kliče po odprtju in inovacijah tistih, ki so teh inovacij sposobni in ki imajo za te aktivnosti tudi primerne finančne in intelektualne vire. Državne strukture so v standardizacijskih postopkih in razvoju postale irelevantne, čeprav se še vedno z veliko silo oklepajo starih prerogativov. V zadnjih dveh desetletjih se je v znanstvene študije, analize in v izpeljavo tehničnih programov nenapovedano in samovoljno vrinila še »politizacija dogovarjanja« na različnih nivojih mednarodnega tehničnega sodelovanja. Ta realnost je dodatno poslabšala razmere v sicer arhaičnih strukturah ITU. Cilj disertacije je nov pristop k mednarodni regulativi uporabe spektra in njegovi mednarodni zaščiti na način, da bo koriščenje frekvenčnega spektra kar najbolj učinkovito in usklajeno. Radiofrekvenčni spekter je univerzalna dobrina, katere uporabnost ni v lasti nikogar in kjer akademske študije in dosežki odpirajo nove dimenzije za njegovo praktično uporabo. V okolju globalnega dogovarjanja in univerzalnega izkoriščanja spektra so pomembni odgovori na spodnja vprašanja, ki nas lahko pripeljejo k novi paradigmi: Kako zagotoviti transparentnost in učinkovitost pri planiranju, razdeljevanju in dodeljevanju frekvenčnega spektra? Kako pritegniti h kreativnemu sodelovanju nove Partnerje (industrija in privatni sektor) in jim zagotoviti pogoje za inovativne rešitve v standardizaciji ICT. Kako ustvariti homogeno in konvergenčno okolje, ki bi zagotavljalo nenehno evolucijo ICT k racionalnim in globalnim usmeritvam pri določanju komunikacijske politike?The convergence of fast developing technologies (ICT), the globalisation of telecommunications in general and availability of new services, have profoundly changed the telecommunication environment and related national policies. The ITU constituencies are Member States and the Organisation strategic decisions are taken exclusively by the Governments. However, in the changing environment it became impossible to ignore the non-governmental partners’ views while discussing the global policies and action plans that should be leading to important decisions. The appearance of strategic alliances, privatization of telecommunication infrastructures and market liberalization are the phenomena that brought new elements in telecommunication development and had substantially modified the role of Governments. These phenomena influenced the Union operational model to great extent where voices of partners are claiming more balanced share in the decision making processes and rational distribution of responsibilities, i.e. between the administrations of Members States, national regulators, telecommunication operators and industry. The necessary transparency of regulatory policy together with simplified Frequency Spectrum Management and universally recognized standardisation activities may be endangered, if no constitutional reforms intervene in the next two or three plenipotentiary cycles. The situation is calling for open review of governmental responsibilities in order to make realistic projection of reforms that should allow ITU to preserve its technical sovereignty and global credibility through its important mission. The ITU structure and its mode of operation in the field of radiocommunications and ICT standardisation today appear to be archaic and not synchronized neither with the environment nor with Members and other user’s needs. National concern for harmonious development of Information and Communication Infrastructure (ICT) remain high on each Government agenda due to its substantial impact on national economies. There is strategic interest to build up and preserve well-coordinated international regulatory regime on Frequency Spectrum/Orbit Management and global/universal and transparent ICT standardisation processes. And the present thesis is a modest contribution to make it happen

Al'tshuler-Aronov-Spivak oscillations of bosonic matter-wave beams in the presence of interaction

We study theoretically the propagation of a guided atom laser across an Aharonov-Bohm ring which is exposed to a synthetic gauge field. The presence of disorder within the ring gives rise to Al'tshuler-Aronov-Spivak oscillations, seen in the disorder average of the transmission as a function of the effective gauge flux that is contained within the ring. Those oscillations are induced by coherent backscattering and represent a manifestation of weak localization. Through analytical and numerical calculations that are based on the mean-field Gross-Pitaevskii approximation for the propagating Bose-Einstein condensate, we show that the presence of a very weak atom-atom interaction within the ring leads to an inversion of the Al'tshuler-Aronov-Spivak oscillations, in a very similar manner as for the coherent backscattering of Bose-Einstein condensates within two-dimensional disorder potentials. Numerical simulations based on the truncated Wigner method reveal that this signature of weak antilocalization becomes washed out if the interaction strength is increased

Influence of grain size, shape and compaction on georadar waves: example of an Aeolian dune

Many Ground Penetrating Radar (GPR) profiles acquired in dry aeolian environment have shown good reflectivity inside present-day dunes. We show that the origin of this reflectivity is related to changes in grain size distribution, packing and/or grain shape in a sandy material. We integrate these three parameters into analytical models for bulk permittivity in order to predict the reflections and the velocity of GPR waves. We consider two GPR cross-sections acquired over Aeolian dunes in the Chadian desert. The 2D migration of GPR data suggests that dunes contain different kinds of bounding surfaces. We discuss and model three kinds of reflections using reasonable geological hypothesis about Aeolian sedimentation processes. The propagation and the reflection of radar waves are calculated using the 1D wavelet modelling method in spectral domain. The results of the forward modelling are in good accordance with real observed data

QR-CODE PROJECT : Towards better traceability of field sampling data

Ensuring traceability of field experimental data or laboratory sampling data to conduct reproducible research is a challenge at the present time. Between the time when geolocalized specimens (biotic or abiotic) are taken, and the time the resulting data ends up in analysis published with a study, many manual operations take place and may generate errors. The French LTSER have joined forces at the national level to propose a solution as generic as possible to this problem of monitoring of the samples and the data associated with them. Compared to existing solutions (such as Laboratory Information Management Systems), we target a robust labeling solution adapted to outdoor working conditions, with the management of stocks and movements of samples. We designed and realized a prototype tested from end to end, using an open source software, cheap Zebra printers (mobile or not) and raspberries as devices. This solution provides sufficient flexibility for the wide variety of existing protocols. In strength is the record of all contextual data associated with the samples, which constitute important parameters of the subsequent analyses. At last, not only traceability is guaranteed, but also a gain of time and a rationalization of the storage of samples that will induce a return on investment

Experimental Induction of Paromomycin Resistance in Antimony-Resistant Strains of L. donovani: Outcome Dependent on In Vitro Selection Protocol

Paromomycin (PMM) has recently been introduced for treatment of visceral leishmaniasis in India. Although no clinical resistance has yet been reported, proactive vigilance should be warranted. The present in vitro study compared the outcome and stability of experimental PMM-resistance induction on promastigotes and intracellular amastigotes. Cloned antimony-resistant L. donovani field isolates from India and Nepal were exposed to stepwise increasing concentrations of PMM (up to 500 µM), either as promastigotes or intracellular amastigotes. One resulting resistant strain was cloned and checked for stability of resistance by drug-free in vitro passage as promastigotes for 20 weeks or a single in vivo passage in the golden hamster. Resistance selection in promastigotes took about 25 weeks to reach the maximal 97 µM inclusion level that did not affect normal growth. Comparison of the IC50 values between the parent and the selected strains revealed a 9 to 11-fold resistance for the Indian and 3 to 5-fold for the Nepalese strains whereby the resistant phenotype was also maintained at the level of the amastigote. Applying PMM pressure to intracellular amastigotes produced resistance after just two selection cycles (IC50 = 199 µM) compared to the parent strain (IC50 = 45 µM). In the amastigote-induced strains/clones, lower PMM susceptibilities were seen only in amastigotes and not at all in promastigotes. This resistance phenotype remained stable after serial in vitro passage as promastigote for 20 weeks and after a single in vivo passage in the hamster. This study clearly demonstrates that a different PMM-resistance phenotype is obtained whether drug selection is applied to promastigotes or intracellular amastigotes. These findings may have important relevance to resistance mechanism investigations and the likelihood of resistance development and detection in the field

Length Sensing and Control in the Virgo Gravitational Wave Interferometer

The gravitational wave detector Virgo is presently being commissioned. A significant part of last year was spent in setting up the cavity length control system. This work was carried out with steps of increasing complexity: locking a simple Fabry-Perot cavity, then a Michelson interferometer with Fabry-Perot cavities in both arms, and finally recycling the light beam into the interferometer. The applied strategy and the main results obtained are describe

The Virgo interferometric gravitational antenna

Submitted to: Class. Quantum Grav.The interferometric gravitational wave detectors represent the ultimate evolution of the classical Michelson interferometer. In order to measure the signal produced by the passage of a gravitational wave, they aim to reach unprecedent sensitivities in measuring the relative displacements of the mirrors. One of them , the 3-km-long Virgo gravitational wave antenna, which will be particularly sensitive in the low frequency range (10-100 Hz), is presently in its commissioning phase. In this paper the various techniques developed in order to reach its target extreme performance are outlined